1. Field of the Invention
The present invention relates to a method and a device for comminuting particulate organic substances in suspensions of microorganisms in a carrier medium, particularly in wastewaters or sludges of biological sewage treatment plants.
2. Description of the Background Art
When wastewater is treated in industrial and municipal biological sewage treatment plants using the so-called activated sludge method, sewage sludge in the form of bacteria suspensions is generated by bacteria due to the metabolism of biodegradable substances. Because this sewage sludge can only be deposited, burned or used in agriculture to a limited degree due to laws and economic constraints, the reduction, or even prevention, of sewage sludge is becoming more and more important.
Over the past few years, the mechanical cell disruption (disintegration) of the organisms in the sewage sludge has been explored as a method for waste reduction. The objective of the disintegration of sewage sludge is to destroy the cell walls of at least a part of the microorganisms contained in the sewage sludge and to release the protoplasts.
Essentially, the objective of this cell disruption is twofold. On the one hand, to improve the anaerobic sludge treatment by accelerated and increased decomposition. The acceleration is based on the mechanical support of hydrolysis because the cell disruption causes the release of the easily degradable intracellular water. Additionally, facultative anaerobic microorganisms are to be disintegrated, which otherwise would partly survive the anaerobic digestion process, and which are in part responsible for the residual content of organic substances in the digested sludge. The cell disruption serves the purpose of exposing them to increased disintegration.
On the other hand, the disintegration is to make it possible to utilize the intracellular water, which contains organic substances like protein and polysaccharide, as an internal carbon source. The objective thereby is to reduce the amount of sludge and the duration of the digestion and to increase the amount of biogas produced for renewable energy. Further advantages are, for example, the destruction of floating sludge and thread bacteria as well as an improvement of the settling properties of the sludges.
A review of the conventional mechanical disintegration methods is given by N. Dichtl, J. Müller, E. Englmann, F. W. Günthert, and M. Osswald in an article “Desintegration von Klärschlamm—ein aktueller Überblick” in “Korrespondenz Abwasser”, 1997 (44), No. 10, pages 1726 to 1738 ff. According to the article, suitable for large-scale technical use are primarily: an attrition ball mill; a high pressure homogenizer; and an ultrasound homogenizer.
Whereas in the attrition ball mill, the cell disruption is caused in a cylindrical milling chamber, which is filled with balls of hard glass or ceramic, by the rotation of the balls, cavitation processes are utilized in the ultrasound and high pressure homogenizers for the disruption of the cells.
All conventional methods for mechanical disintegration have in common that monetary and energy expenditures for generating the cavitation processes, which generate the forces that cause a splitting of the cell walls of the microorganisms, are very high. That is true for both the manufacture and the operation and maintenance of high pressure and ultrasound homogenizers. Whereas with high pressure homogenizers, very high pressures have to be generated, which require a very high pump capacity, the ultrasound method requires a great amount of electrical energy to feed the sonotrodes. A further disadvantage of the utilization of cavitation features in this context is that delamination of equipment and materials occur, which is why expensive materials, for example titanium, have to be used, particularly for wear-and-tear intensive components such as the ultrasound sonotrodes.
To reduce the expenditures in energy and equipment for generating cavitation features, it has already been suggested in the older, not published German patent application according to DE 102 14 689 A1, not to utilize equipment and energy-technically expensive ultrasound or high pressure disintegrators for the disruption of organic substances, but instead to convey the suspension under pressure via a nozzle having initially a narrow cross-section and then an expanding cross-section, a so-called Lavel nozzle. By narrowing of the cross-section, the flow speed of the suspension is accelerated such that the pressure falls to a level below the vapor pressure of the carrier substance, namely water, whereas, when passing through the subsequently expanding cross-section, collapsing cavitation bubbles are generated as a result of pressure compensation.
Although the treatment of suspensions such as these through methods with cavitation features has met with success to some degree, that is, it is successful as far as a higher yield of biogas and a reduction of the sludge share is concerned, however, it cannot be clarified without a doubt if these results are due to shear stress in the flow channel, the generation and implosion of the cavitation bubbles, or other effects, which up to now could not be clarified beyond a doubt. In any case, the forming and collapsing of the cavitation bubbles in an implosion-like manner has basically a local effect only; it does not primarily cause the destruction of the cells, and thus the disintegration of the microorganisms.